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LDAP backend to slapd




The LDAP backend to slapd(8) is not an actual database; instead it acts as a proxy to forward incoming requests to another LDAP server. While processing requests it will also chase referrals, so that referrals are fully processed instead of being returned to the slapd client.

Sessions that explicitly Bind to the back-ldap database always create their own private connection to the remote LDAP server. Anonymous sessions will share a single anonymous connection to the remote server. For sessions bound through other mechanisms, all sessions with the same DN will share the same connection. This connection pooling strategy can enhance the proxy's efficiency by reducing the overhead of repeatedly making/breaking multiple connections.

The ldap database can also act as an information service, i.e. the identity of locally authenticated clients is asserted to the remote server, possibly in some modified form. For this purpose, the proxy binds to the remote server with some  administrative identity, and, if required, authorizes the asserted identity. See the idassert-* rules below. The administrative identity of the proxy, on the remote server, must be  allowed to authorize by means of appropriate authzTo rules; see slapd.conf(5) for details.

The proxy instance of slapd(8) must contain schema information for the attributes and objectClasses used in filters, request DNs and request-related data in general. It should also contain schema information for the data returned by the proxied server. It is the responsibility of the proxy administrator to keep the schema of the proxy lined up with that of the proxied server.

Note: When looping back to the same instance of slapd(8), each connection requires a new thread; as a consequence, the slapd(8) threads parameter may need some tuning. In those cases, one may consider using slapd-relay(5) instead, which performs the relayed operation  internally and thus reuses the same connection.


These slapd.conf options apply to the LDAP backend database. That is, they must follow a "database ldap" line and come before any subsequent "backend" or "database" lines. Other database options are described in the slapd.conf(5) manual page.

Note: In early versions of back-ldap it was recommended to always set

lastmod  off

for ldap and meta databases. This was required because operational attributes related to entry creation  and modification should not be proxied, as they could be mistakenly written to the target server(s), generating an error. The current implementation automatically sets lastmod to off,  so its use is redundant and should be omitted.

uri <ldapurl>

LDAP server to use.  Multiple URIs can be set in a single ldapurl argument, resulting in the underlying library automatically  calling the first server of the list that responds, e.g.

uri "ldap://host/ ldap://backup-host/"

The URI list is space- or comma-separated. Whenever the server that responds is not the first one in the list, the list is rearranged and the responsive server is moved to the head, so that it will be first contacted the next time a connection needs to be created.

acl-bind bindmethod=simple|sasl [binddn=<simple DN>] [credentials=<simple password>] [saslmech=<SASL mech>] [secprops=<properties>] [realm=<realm>] [authcId=<authentication ID>] [authzId=<authorization ID>] [starttls=no|yes|critical] [tls_cert=<file>] [tls_key=<file>] [tls_cacert=<file>] [tls_cacertdir=<path>] [tls_reqcert=never|allow|try|demand] [tls_reqsan=never|allow|try|demand] [tls_cipher_suite=<ciphers>] [tls_ecname=<names>] [tls_protocol_min=<major>[.<minor>]] [tls_crlcheck=none|peer|all]

Allows one to define the parameters of the authentication method that is internally used by the proxy to collect info related to access control, and whenever an operation occurs with the identity of the rootdn of the LDAP proxy database. The identity defined by this directive, according to the properties associated to the authentication method, is supposed to have read access  on the target server to attributes used on the proxy for ACL checking.

There is no risk of giving away such values; they are only used to check permissions. The default is to use simple bind, with empty binddn and credentials, which means that the related operations will be performed anonymously. If not set, and if idassert-bind is defined, this latter identity is used instead.  See idassert-bind for details.

The connection between the proxy database and the remote server associated to this identity is cached regardless of the lifespan of the client-proxy connection that first established it.

This identity is not implicitly used by the proxy when the client connects anonymously. The idassert-bind feature, instead, in some cases can be crafted to implement that behavior, which is intrinsically unsafe and should be used with extreme care.

The TLS settings default to the same as the main slapd TLS settings, except for tls_reqcert which defaults to "demand", and tls_reqsan which defaults to "allow".

cancel {ABANDON|ignore|exop[-discover]}

Defines how to handle operation cancellation. By default, abandon is invoked, so the operation is abandoned immediately. If set to ignore, no action is taken and any further response is ignored; this may result in further response messages to be queued for that connection, so it is recommended that long lasting connections are timed out either by idle-timeout or conn-ttl, so that resources eventually get released. If set to exop, a cancel operation (RFC 3909) is issued, resulting in the cancellation  of the current operation; the cancel operation waits for remote server response, so its use  may not be recommended. If set to exop-discover, support of the cancel extended operation is detected by reading the remote server's root DSE.

chase-referrals {YES|no}

enable/disable automatic referral chasing, which is delegated to the underlying libldap, with rebinding eventually performed if the rebind-as-user directive is used.  The default is to chase referrals.

conn-pool-max <int>

This directive defines the maximum size of the privileged connections pool.

conn-ttl <time>

This directive causes a cached connection to be dropped after a given ttl, regardless of being idle or not.  If a client connection outlives the remote connection, the client will receive LDAP_UNAVAILABLE when it executes the next operation.

idassert-authzFrom <authz-regexp>

if defined, selects what local identities are authorized to exploit the identity assertion feature. The string <authz-regexp> mostly follows the rules defined for the authzFrom attribute. See slapd.conf(5), section related to authz-policy, for details on the syntax of this field.  This parameter differs from the documented behavior in relation to the meaning of *, which in this case allows anonymous rather than denies.

idassert-bind bindmethod=none|simple|sasl [binddn=<simple DN>] [credentials=<simple password>] [saslmech=<SASL mech>] [secprops=<properties>] [realm=<realm>] [authcId=<authentication ID>] [authzId=<authorization ID>] [authz={native|proxyauthz}] [mode=<mode>] [flags=<flags>] [starttls=no|yes|critical] [tls_cert=<file>] [tls_key=<file>] [tls_cacert=<file>] [tls_cacertdir=<path>] [tls_reqcert=never|allow|try|demand] [tls_reqsan=never|allow|try|demand] [tls_cipher_suite=<ciphers>] [tls_ecname=<names>] [tls_protocol_min=<version>] [tls_crlcheck=none|peer|all]

Allows one to define the parameters of the authentication method that is internally used by the proxy to authorize connections that are  authenticated by other databases. Direct binds are always proxied without any idassert handling.

The identity defined by this directive, according to the properties associated to the authentication method, is supposed to have auth access  on the target server to attributes used on the proxy for authentication and authorization, and to be allowed to authorize the users. This requires to have proxyAuthz privileges on a wide set of DNs, e.g. authzTo=dn.subtree:"", and the remote server to have authz-policy set to to or both. See slapd.conf(5) for details on these statements and for remarks and drawbacks about their usage. The supported bindmethods are


where none is the default, i.e. no identity assertion is performed.

The authz parameter is used to instruct the SASL bind to exploit native SASL authorization, if available; since connections are cached, this should only be used when authorizing with a fixed identity (e.g. by means of the authzDN or authzID parameters). Otherwise, the default proxyauthz is used, i.e. the proxyAuthz control (Proxied Authorization, RFC 4370) is added to all operations.

The supported modes are:

<mode> := {legacy|anonymous|none|self}

If <mode> is not present, and authzId is given, the proxy always authorizes that identity. <authorization ID> can be



The former is supposed to be expanded by the remote server according  to the authz rules; see slapd.conf(5) for details. In the latter case, whether or not the dn: prefix is present, the string must pass DN validation and normalization.

The default mode is legacy, which implies that the proxy will either perform a simple bind as the authcDN or a SASL bind as the authcID and assert the client's identity when it is not anonymous. The other modes imply that the proxy will always either perform a simple bind  as the authcDN or a SASL bind as the authcID, unless restricted by idassert-authzFrom rules (see below), in which case the operation will fail; eventually, it will assert some other identity according to <mode>. Other identity assertion modes are anonymous and self, which respectively mean that the empty or the client's identity will be asserted; none, which means that no proxyAuthz control will be used, so the authcDN or the authcID identity will be asserted. For all modes that require the use of the proxyAuthz control, on the remote server the proxy identity must have appropriate authzTo permissions, or the asserted identities must have appropriate authzFrom permissions.  Note, however, that the ID assertion feature is mostly  useful when the asserted identities do not exist on the remote server.

Flags can be


When the override flag is used, identity assertion takes place even when the database is authorizing for the identity of the client, i.e. after binding with the provided identity, and thus authenticating it, the proxy performs the identity assertion using the configured identity and authentication method.

When the prescriptive flag is used (the default), operations fail with inappropriateAuthentication for those identities whose assertion is not allowed by the idassert-authzFrom patterns. If the non-prescriptive flag is used, operations are performed anonymously for those identities  whose assertion is not allowed by the idassert-authzFrom patterns.

When the proxy-authz-non-critical flag is used (the default), the proxyAuthz control is not marked as critical, in violation of RFC 4370.  Use of proxy-authz-critical is recommended.

When the dn-authzid flag is used, RFC 3829 LDAP Authorization Identity Controls is used to retrieve the identity associated to the SASL identity; when the dn-whoami flag is used, RFC 4532 LDAP Who am I? Operation is performed after the bind for the same purpose.

The TLS settings default to the same as the main slapd TLS settings, except for tls_reqcert which defaults to "demand", and tls_reqsan which defaults to "allow".

The identity associated to this directive is also used for privileged operations whenever idassert-bind is defined and acl-bind is not.  See acl-bind for details.

idassert-passthru <authz-regexp>

if defined, selects what local identities bypass the identity assertion feature. Those identities need to be known by the remote host. The string <authz-regexp> follows the rules defined for the authzFrom attribute. See slapd.conf(5), section related to authz-policy, for details on the syntax of this field.

idle-timeout <time>

This directive causes a cached connection to be dropped after it has been idle for the specified time.  If a client connection outlives the remote connection, the client will receive LDAP_UNAVAILABLE when it executes the next operation.

keepalive <idle>:<probes>:<interval>

The keepalive parameter sets the values of idle, probes, and interval used to check whether a socket is alive; idle is the number of seconds a connection needs to remain idle before TCP starts sending keepalive probes; probes is the maximum number of keepalive probes TCP should send before dropping the connection; interval is interval in seconds between individual keepalive probes. Only some systems support the customization of these values; the keepalive parameter is ignored otherwise, and system-wide settings are used.

tcp-user-timeout <milliseconds>

If non-zero, corresponds to the TCP_USER_TIMEOUT set on the target connections, overriding the operating system setting. Only some systems support the customization of this parameter, it is ignored otherwise and system-wide settings are used.

network-timeout <time>

Sets the network timeout value after which poll(2)/select(2) following a connect(2) returns in case of no activity. The value is in seconds, and it can be specified as for idle-timeout.

norefs <NO|yes>

If yes, do not return search reference responses. By default, they are returned unless request is LDAPv2.

omit-unknown-schema <NO|yes>

If yes, do not return objectClasses or attributes that are not known to the local server. The default is to return all schema elements.

noundeffilter <NO|yes>

If yes, return success instead of searching if a filter is undefined or contains undefined portions. By default, the search is propagated after replacing undefined portions with (!(objectClass=*)), which corresponds to the empty result set.

onerr {CONTINUE|stop}

This directive allows one to select the behavior in case an error is returned by the remote server during a search. The default, continue, consists in returning success. If the value is set to stop, the error is returned to the client.

protocol-version {0,2,3}

This directive indicates what protocol version must be used to contact the remote server. If set to 0 (the default), the proxy uses the same protocol version  used by the client, otherwise the requested protocol is used. The proxy returns unwillingToPerform if an operation that is  incompatible with the requested protocol is attempted.

proxy-whoami {NO|yes}

Turns on proxying of the WhoAmI extended operation. If this option is given, back-ldap will replace slapd's original WhoAmI routine with its own. On slapd sessions that were authenticated by back-ldap, the WhoAmI request will be forwarded to the remote LDAP server. Other sessions will be handled by the local slapd, as before. This option is mainly useful in conjunction with Proxy Authorization.

quarantine <interval>,<num>[;<interval>,<num>[...]]

Turns on quarantine of URIs that returned LDAP_UNAVAILABLE, so that an attempt to reconnect only occurs at given intervals instead of any time a client requests an operation. The pattern is: retry only after at least interval seconds elapsed since last attempt, for exactly num times; then use the next pattern. If num for the last pattern is "+", it retries forever; otherwise,  no more retries occur. The process can be restarted by resetting the olcDbQuarantine attribute of the database entry in the configuration backend.

rebind-as-user {NO|yes}

If this option is given, the client's bind credentials are remembered for rebinds, when trying to re-establish a broken connection, or when chasing a referral, if chase-referrals is set to yes. Note, however, that connection is not re-established automatically after it was dropped due to idle-timeout or conn-ttl .

session-tracking-request {NO|yes}

Adds session tracking control for all requests. The client's IP and hostname, and the identity associated to each request, if known, are sent to the remote server for informational purposes. This directive is incompatible with setting protocol-version to 2.

single-conn {NO|yes}

Discards current cached connection when the client rebinds.

t-f-support {NO|yes|discover}

enable if the remote server supports absolute filters (see RFC 4526 for details). If set to discover, support is detected by reading the remote server's root DSE.

timeout [<op>=]<val> [...]

This directive allows one to set per-operation timeouts. Operations can be

<op> ::= bind, add, delete, modrdn, modify, compare, search

The overall duration of the search operation is controlled either by the timelimit parameter or by server-side enforced time limits (see timelimit and limits in slapd.conf(5) for details). This timeout parameter controls how long the target can be  irresponsive before the operation is aborted. Timeout is meaningless for the remaining operations, unbind and abandon, which do not imply any response, while it is not yet implemented in currently supported extended  operations. If no operation is specified, the timeout val affects all supported operations.

Note: if the timelimit is exceeded, the operation is cancelled (according to the cancel directive); the protocol does not provide any means to rollback operations, so the client will not be notified about the result of the operation, which may eventually succeeded or not. In case the timeout is exceeded during a bind operation, the connection is destroyed, according to RFC4511.

Note: in some cases, this backend may issue binds prior to other operations (e.g. to bind anonymously or with some prescribed identity according to the idassert-bind directive). In this case, the timeout of the operation that resulted in the bind is used.

tls {none|[try-]start|[try-]propagate|ldaps} [starttls=no] [tls_cert=<file>] [tls_key=<file>] [tls_cacert=<file>] [tls_cacertdir=<path>] [tls_reqcert=never|allow|try|demand] [tls_reqsan=never|allow|try|demand] [tls_cipher_suite=<ciphers>] [tls_ecname=<names>] [tls_crlcheck=none|peer|all]

Specify TLS settings for regular connections.

If the first parameter is not "none" then this configures the TLS settings to be used for regular connections. The StartTLS extended operation will be used when establishing the connection unless the URI directive protocol scheme is ldaps://. In that case this keyword may only be set to "ldaps" and the StartTLS operation will not be used.

With propagate, the proxy issues the StartTLS operation only if the original connection has a TLS layer set up. The try- prefix instructs the proxy to continue operations if the StartTLS operation failed; its use is not recommended.

The TLS settings default to the same as the main slapd TLS settings, except for tls_reqcert which defaults to "demand", tls_reqsan which defaults to "allow", and starttls which is overshadowed by the first keyword and thus ignored.

use-temporary-conn {NO|yes}

when set to yes, create a temporary connection whenever competing with other threads for a shared one; otherwise, wait until the shared connection is available.

Access Control

The ldap backend does not honor all ACL semantics as described in slapd.access(5). In general, access checking is delegated to the remote server(s). Only read (=r) access to the entry pseudo-attribute and to the other attribute values of the entries returned by the search operation is honored, which is performed by the frontend.


The LDAP backend provides basic proxying functionalities to many overlays. The chain overlay, described in slapo-chain(5), and the translucent overlay, described in slapo-translucent(5), deserve a special mention.

Conversely, there are many overlays that are best used in conjunction with the LDAP backend. The proxycache overlay allows caching of LDAP search requests (queries)  in a local database. See slapo-pcache(5) for details. The rwm overlay provides DN rewrite and attribute/objectClass mapping capabilities to the underlying database. See slapo-rwm(5) for details.



default slapd configuration file

See Also

slapd.conf(5), slapd-config(5), slapd-meta(5), slapo-chain(5), slapo-pcache(5), slapo-rwm(5), slapo-translucent(5), slapd(8), ldap(3).


Howard Chu, with enhancements by Pierangelo Masarati

Referenced By

slapd-asyncmeta(5), slapd.backends(5), slapd-meta(5), slapd.overlays(5), slapo-chain(5), slapo-pbind(5), slapo-pcache(5), slapo-rwm(5), slapo-translucent(5).

2024/01/29 OpenLDAP 2.6.7